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Soluble organic molecules with diverse and highly tunable structures are receiving more and more attention as redox‐active materials for aqueous organic flow batteries (AOFBs). However, the undesirable parasitic reactions of redox‐active molecules restrict their practical application. Herein, a general molecular stabilization strategy is proposed by introducing amino groups on 3,7‐positions of polycyclic azine derivatives (phenothiazine and phenoxazine compounds) to effectively enhance the redox activity and electrochemical stability of molecules. The theoretical calculation and a series of electrochemical analyses revealed that the 3,7‐substituted amino groups serving as the redox active sites can stabilize the intermediate radicals by delocalizing the electron density in the acidic solution, preventing nucleophilic/electrophilic attack. As a result, the designed 3,7‐bis‐((2‐hydroxyethyl)(methyl)amino)phenothiazin‐5‐ium bromide not only achieved the remarkable electrochemical cycling performance in the flow cell with an ultra‐low‐capacity fade rate of 0.00029% per cycle for 18000 cycles (62 days) and but also delivered a stable high capacity of 47 Ah L−1 (with capacity fade 0.0056% per cycle, 0.077% per day) under ambient condition. Such finding undoubtedly provides a guidance to design stable redox‐active molecules for AOFBs. [ABSTRACT FROM AUTHOR]

Due to decoupled energy and power, the aqueous organic redox flow battery (AORFB) represents a promising energy storage technology that stores energy in redox‐active organic compounds dissolved in aqueous electrolytes. The organic compounds are composed of earth‐abundant elements and are therefore potentially cost‐effective. Their structures are diverse and highly tunable, rendering it possible to regulate the redox potential, water solubility, and cycling lifespan. Therefore, the adoption of redox‐active organics greatly expands the space for enabling exceeding performance merits compared to their inorganic counterparts. Herein, this work reviews the molecular design and engineering scheme of organic electrolytes, focusing exclusively on pH‐neutral AORFB that uses non‐corrosive and nonflammable pH‐neutral aqueous electrolytes with inexpensive inorganic salts as supporting electrolytes and features high safety, low corrosivity, low cost, and environmental benignity. Some comments on the present challenges and prospects of this ascendant domain are also presented. [ABSTRACT FROM AUTHOR]

Abstract: The disproportionation of polysulfide (PS) is a long‐neglected and vital issue that causes the fast capacity fading of Li–S batteries. Based on the hard and soft acids and bases (HSAB) theory, a large size N‐methyl‐N‐ethyl pyrrolidinium (MEP+) cation is proposed to complex and stabilize the PS in electrolyte. The disproportionation of PS is successfully suppressed by this simple method, thereby avoiding the precipitation of sulfur in the electrolyte and reducing the loss of the active materials. The mutual interaction mechanism between MEP+ and Sn2− in electrolyte is comprehensively investigated and verified for the first time, via both density functional theory (DFT) calculation and experimental characterization. It enables the 5000 mA h Li–S batteries (soft package type) to achieve initial specific energy over 300 Wh kg−1 and maintain over 65% after 100 charge/discharge cycles at 1/20 C, while merely 24% is remained at 59 cycles without MEP+. This interesting finding is believed to shed light on the further development of Li–S batteries. [ABSTRACT FROM AUTHOR]

Porous membranes with critically hydrophobic/hydrophilic phase-separated-like structures for use in vanadium flow battery application are first realized by solvent-induced reassembly of a polymer blend system. Porous poly(ether sulfone) (PES)/sufonated poly(ether ether ketone) (SPEEK) blend membranes with tunable pore size are prepared via the phase inversion method. After solidification, isopropanol (IPA) is introduced to induce the reassembly of sulfonated groups and further form ion-transport channels by using the interaction between IPA and functional groups in SPEEK. As a result, a highly phase separated membrane structure is created, composed of a highly stable hydrophobic porous PES matrix and hydrophilic interconnected small pores. The charged pore walls are highly beneficial to improving proton conductivity, while pores are simultaneously shrunk during the IPA treatment. Therefore, the resultant membranes show an excellent battery performance with a coulombic efficiency exceeding 99%, along with an energy efficiency over 91%, which is among the highest values ever reported. This article supplies an ease-to-operate and efficient method to create membranes with controlled ion-transport channels. [ABSTRACT FROM AUTHOR]

Advanced charged sponge-like porous membranes with ultrahigh stability and selectivity are designed and fabricated for vanadium flow battery (VFB) applications. The designed porous membranes are fabricated via constructing positively charged cross-linked networks on the pore walls of polysulfone membranes. The charge density of the pore walls can be tuned by changing the crosslinking time. The positively charged pore walls can effectively retain vanadium ions via Donnan exclusion, hence keeping extremely high selectivity, while the crosslinked network effectively increases the membrane stability. As a result, the designed membranes exhibit an outstanding performance, combining extremely high selectivity and stability. The single cell assembled with the prepared porous membrane shows a columbic efficiency of 99% and an energy efficiency of 86% at a current density of 80 mA cm−2, which is much higher than Nafion 115 (93.5%; 82.3%). A battery assembled with the prepared membrane shows a stable battery performance over more than 6000 cycles, which is by far the longest record for porous membranes ever reported. These results indicate that advanced, charged, sponge-like, porous membranes with a crosslinked pore-wall structure are highly promising for VFB applications. [ABSTRACT FROM AUTHOR]